1. Field Of The Invention
The field of the invention is an automatically operated, thermally responsive, current disconnecting circuit for a heating device. Circuits of this type are commonly referred to as heater shutdown circuits. Circuits and devices of this type are often found in Class 219, Subclass 510 and adjacent subclasses.
Most specifically, the present invention relates to a heater shutdown circuit which is operative to shut off current to an electric heating element when the resistance thereof increases due to the overheating thereof and is operative to maintain the current shut off even after the heating element cools.
The heater shutdown circuit is designed so that the heating element cannot be re-energized after the current has been shut off until the heater shutdown circuit has been de-energized for a predetermined period of time, after which the heater shutdown circuit can be re-connected to the current source to again energize the heating element.
2. Description Of The Prior Art
Heretofore, various control circuits have been proposed for controlling the current to an electric heating element in response to the temperature of the heating element. Examples of such control circuits are disclosed in the following U.S. Pat. Nos. 2,549,095; 2,722,595; 3,448,245; 3,546,436 and 3,614,392.
These previously proposed circuits have been operative either to shut off the current to the heating element completely after the temperature of the heating element exceeds a certain level or to shut off and turn on a heating element intermittently to maintain the temperature of the heating element within a preselected range. Typically, these control circuits are operative to re-energize the heating element once the heating element has cooled to a temperature below the preselected temperature. When there is a malfunction in the electric heater such that the heating element continues to overheat, the control circuit will cycle continuously, turning off and turning on the flow of current to the heating element. Although this type of operation is acceptable in many environments, it is not usually acceptable when the heating element is utilized for heating a liquid such as water in a hot water heater or cup or such as blood in a dialysis machine.
Accordingly, it is desirable to provide a heater shutdown circuit which will maintain a heating element shut off even after the heating element cools to a temperature below the temperature at which the circuit is actuated to turn off the flow of current to the heating element. One prior art circuit which utilizes transformers and relays and which has a secondary control to maintain an electric blanket de-energized after a malfunction occurs therein is disclosed in U.S. Pat. No. 2,549,095.
As will be described hereinafter in greater detail, the heater shutdown circuit of the present invention provides a circuit which utilizes electronic switching elements and which functions as follows:
(1) Initially passes current to the heating element;
(2) Shuts off the flow of current to the heating element once the temperature of the heating element exceeds a predetermined temperature;
(3) Maintains the flow of current to the heating element shut off after the heating element cools to a temperature below the predetermined temperature; and
(4) Requires de-energization of the heater shutdown circuit for a predetermined time after which the heater shutdown circuit can be re-energized to re-energize the heating element.
In the prior art, electronic switching elements of the type utilized in the heater shutdown circuit of the present invention have been utilized in circuits for controlling the flow of current to various loads in electrical circuits. Examples of the use of electronic switching elements, such as transistors and triacs, for controlling the flow of current to a load can be found in the following U.S. Pat. Nos. 3,414,789; 3,566,198; 3,715,651; 3,742,337; 3,745,382; 3,760,157; 3,870,904 and 3,909,703.
These prior art control circuits utilizing electronic switching devices have not, however, provided a heater shutdown circuit having the particular circuit elements and circuit connections of the heater shutdown circuit of the present invention described more fully hereinafter. Moreover, such prior art control circuits do not operate in the same manner as the heater shutdown circuit of the present invention, which is operative, first, to energize a heating element, second, to shut off current to the heating element when the heating element exceeds a predetermined temperature, third, to maintain the flow of current to the heating element shut off after the heating element cools to a temperature below the predetermined temperature and fourth, to prevent re-energization of the heating element until the heater shutdown circuit is de-energized for a predetermined period of time after which it is again operable to energize a heating element.